In the production of sheets and films of thermoplastics material it is known to use extruders provided with flat extruder heads having an extruder slot defined between two lips. Currently, such extruder heads are supplied with material to be extruded through a single duct leading to a point mid way along the length of the slot. Consequently, when heads of a very considerable width are used it is practically impossible, given the differences in flow rate of the material at the points laterally spaced of the outlet of the feed duct to obtain sheets or films having a constant thickness.
There are, indeed, provided various structural arrangements acting to minimise the said dimensional variations in the manufactured product. Such arrangements, however, are in general concerned with means for making mechanical adjustments, directed at modifying the mutual separation of the two lips defining the plastics material extrusion slot. Moreover, such mechanical adjustment means only allow operation of the extruder by trial and error methods and consequently do not permit the slot to be arranged in such a way that it guarantees the uniformity of the product obtained in all circumstances.
Alternatively, it is known to provide heater means acting in cooperation with the said mechanical slot width adjustments means, or instead of such means. Extrusion heads may also have various differentiated heating zones acting to correct possible differences in the flow of the material. However, adjustment of the head is extremely laborious and it can be very difficult to obtain the desired result. When differentiated heating of the various zones of the slot is employed, moreover, it is necessary to provide electronic apparatus of considerable cost and, in general, great delicacy.